Demand for a small light-weight actuator having high flexibility is increasing in the fields of medical instruments, equipment for industrial use, personal robot, micromachine, and the like.
In the case of a small size actuator, actuators of electrostatic attraction type, piezoelectric type, ultrasonic wave type, shape memory alloy type and polymer expansion/contraction type, and electrochemical type actuators such as electron-conducting polymer actuator and ion-conducting polymer actuator using an electrochemical reaction are proposed.
In these actuators, in order to extend applications of small size actuators more, electrochemical polymer actuators which can be driven at low voltage, exhibits a quick response, has high flexibility, can easily realize a small size and a light weight, and can be operated with small electric power have been developed. These actuators are roughly classified into two types, i.e., one is an actuator making use of expansion/contraction by reduction/oxidation of an electron conducting polymer such as polypyrrole and polyaniline in an electrolyte (electron-conducting polymer actuator), and another one is an actuator comprising an ion-exchange membrane and a junction electrode and being capable of functioning as an actuator by applying an electric potential difference to the ion-exchange membrane being in a water-containing state to cause flection or deformation of the ion-exchange membrane (ion-conducting polymer actuator).
Among these actuators, electron-conducting polymer actuators have advantages that they can be operated at low voltage, a degree of expansion/contraction is large and a generated pressure is high, but a response speed is slow, and a preparation process of polypyrrole having most satisfactory performance is limited only to electrolytic polymerization. In addition, it is pointed out that they have a problem with durability for repeated use from theoretical point of view because response depends on doping and de-doping of ion resulting from a redox reaction.
In order to overcome these problems, an actuator having an electrode which is prepared by molding carbon nanotube into a paper form and makes use of an expansion/contraction phenomenon due to a change in interfacial stress resulting from charging and discharging of an electric double layer is proposed (cf. Science, Vol. 284, 1999, p. 1340). This actuator exhibits a quick response and has a long service life due to the principle based on charging and discharging of a double layer. Also it is known that a generated pressure is high. However its degree of expansion/contraction is small, and in its preparation process, a very complicated filtration operation taking a long period of time is necessary. In addition, this actuator has low mechanical strength and is operated only in an electrolytic solution.
On the other hand, any of conventional electron-conducting polymer actuators or ion-conducting polymer actuators has been used mainly in an aqueous solution of electrolyte because an electrolyte is necessary for operation thereof. Ion-conducting polymer actuators are used basically in water because sufficient ion conductivity is not exhibited unless an ion-exchange resin is in a state of being swelled by water. In order to use these actuators in air, it is necessary to prevent evaporation of water. For that purpose, a resin coating method is reported, but in this method, complete coating is difficult and a coating is broken even by generation of a small amount of gas due to an electrode reaction, and further the coating itself becomes a resistance to response deformation. Thus the method has not been put into practical use. Also organic solvents having a high boiling point such as propylene carbonate are used instead of water, but also in this case, there are similar problems, and in addition, there are problems that such solvents are not so high in ion conductivity as water and are inferior in responsivity.
Such being the case, since conventional actuators are driven only in limited environment, mainly in an electrolyte solution, application thereof is so limited. Therefore development of an actuator element driven in air is essential for putting a small size actuator into practical use in a wide range of applications.
For the purpose of operating actuators in air, there are examples of disposing an electron-conducting polymer at both sides of an ion-exchange resin or disposing an conductive polymer on a gel membrane containing an organic solvent having a high boiling point such as propylene carbonate to utilize expansion/contraction of electrodes disposed at both sides and to use as an actuator element. In these examples, there are problems with drying of a solvent and low ion conductivity like the case of ion-conducting polymer actuators, and substantial solutions have not been given.
In order to solve these problems, research for application is recently being advanced with respect to use of salts which are known as ionic liquids, are called normal temperature molten salts or simply molten salts, and show a molten state in a wide temperature range including normal temperature (room temperature). In the case of an ionic liquid, since a vapor pressure can be neglected, drying of a solvent by evaporation can be prevented.
For the purpose of operating electron-conducting polymer actuators in air, there are researches of an expansion/contraction phenomenon of a conductive polymer in an ionic liquid (cf. Science, Vol. 297, 2002, p. 983) and researches of a totally solid-state element using a complex comprising polypyrrole and an ionic liquid of polyvinylidene fluoride (cf. Electrochimica Acta, Vol. 48, 2003, p. 2355). However in these researches, too, theoretical problems as mentioned above which result from a conductive polymer, namely problems with slow response, preparation process and service life have not yet been solved.
In order to overcome these un-solved problems, JP2005-176428A proposes an actuator element which can be operated in air or in vacuo by using a gel of carbon nanotube and ionic liquid as a conductive expandable active layer and using, as a conducting layer, a layer comprising an ionic liquid and a fluorine-containing polymer such as a vinylidene fluoride/hexafluoropropylene copolymer, polyvinylidene fluoride or perfluorosulfonic acid (NAFION, trade mark of Du Pont).
However, in order to improve durability for repeated use of an actuator element, it is necessary to further enhance electrochemical stability and mechanical strength, and further, a resin having a high degree of expansion/contraction and generating a large pressure is demanded.